Atmospheric Science

P.16 M.P. McCormick, R.E. Veiga, "SAGE II measurements of early Pinatubo aerosols," Geophysical Research Letters, 19:155-8, 1992. M. Patrick McCormick (National Aeronautics and Space Administration, Langley Research Center, Hampton, Va.): "Since 1979, the global view of aerosols in the stratosphere and upper troposphere has been provided almost exclusively by the Stratospheric Aerosol Measurement II (SAM II) and the Stratospheric Aerosol and Gas Experiment (SAGE I and II) series of satellite instruments. The resulting data sets have shown that naturally occurring atmospheric aerosols (sub-micron-sized particles composed mainly of aqueous sulfuric acid) are not uniformly distributed, but rather undergo large changes resulting from volcanic eruptions and in response to atmospheric dynamics, microphysical growth, and gravitational sedimentation. A specialized type of `aerosol' also appears in the form of polar stratospheric clouds (PSCs) inside the Antarctic vortex during the cold polar night. These clouds provide catalytic surfaces upon which heterogeneous reactions free reactive chlorine atoms that destroy ozone and create the Antarctic `ozone hole.' "If their global abundance is great enough, as has been the case since the June 1991 eruption of Mount Pinatubo in the Philippines, the aerosols can cool the Earth's surface by reflecting incoming solar radiation and heat the local stratosphere by absorbing upwelling infrared radiation. Changes in the atmospheric temperature structure can, in turn, affect atmospheric dynamical processes. In addition, heterogeneous chemistry similar to that which creates the ozone hole may occur at all latitudes. "Our paper describes the initial perturbation to stratospheric aerosols caused by the eruption of Pinatubo. We show SAGE II- derived maps of its global dispersal at various altitudes and estimate the mass of new aerosols produced. These data show that the Pinatubo perturbation was the largest of the century, perhaps the largest since the 1883 eruption of Krakatoa. "Besides producing beautiful and prolonged sunrises and sunsets, the Pinatubo aerosols are having a significant effect on climate, ozone chemistry, atmospheric dynamics, remote sensor performance, and polar-route aircraft flying through the layer. The enormous amount of aerosol produced--greater than 30 million tons--is predicted by climate modelers to produce a global mean cooling near the surface of about 0.5 C. This amount of cooling is about three times larger than the standard deviation of global mean temperature and, therefore, provides an `acid test' for global climate models. Such cooling will mask for a few years the warming caused by the greenhouse effect. "Similarly, this amount of aerosol increases the global aerosol surface area density by an order of magnitude or more over pre- eruption stratospheric values and, therefore, greatly increases the probability of perturbations to `normal' homogeneous ozone photochemistry. Perhaps more important, it provides this enhanced aerosol surface area not just in the polar regions, but globally, with increased potential for widespread stratospheric ozone depletion. The aerosol level also produces artifacts or reduced accuracy in the data retrieved by satellite-based and ground- based remote sensors, which must be taken into account in interpreting the measurements. "Finally, since the cruise altitude for aircraft flying on polar routes is in the lower stratosphere, the surfaces of these aircraft come in contact with this new sulfuric-acid aerosol. One damaging effect is a phenomenon called crazing, where-by microscopic cracks are produced in the outer layer of the passenger windows made of acrylic, making it difficult or impossible to view out of them. "This paper and subsequent data since publication are providing these important global data to researchers in each of the fields mentioned previously and, in addition, have contributed to an understanding of the gas-to-particle conversion processes, the particle condensation and growth processes, and various atmospheric dynamical processes."

Atmospheric Science

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